Curable silicone gums are normally compounded with reinforcing fillers, such as finely divided silica, to impart maximum physical properties when cured into rubber. To preclude or control crepe hardening during shelf aging, i.e. preventing polymer-filler interactions, the filler is surface treated with an agent octamethylcyclotetrasiloxane or a fluorosilicone hydrolyzate to reduce the number of free silanol groups on the surface of the finely divided silica.
One method for surface treating the silica filler, U.S. Pat. No. 4,529,774, exposes the same to a fluorosilicone hydrolyzate in the vapor phase. The hydrolyzate is a mixture of silanol terminated linear siloxane oligomers with mixed cyclic siloxane compounds. The ratio of linears to cyclics varies with the linears being 14-36% by weight while the cyclics may vary over the range of 64-86% by weight. The hydroxy end-group content for such a hydrolyzate may vary over the range of 1.0-2.5% by weight.
The vapor phase filler treatment involves an initial heating of the silica in a fluidized reactor at temperatures in excess of 110.degree. C. to effect removal of adsorbed moisture. The hydrolyzate is then pumped in, under pressure, while the temperature is raised to 280.degree.-300.degree. C. to effect the desired grafting to surface silanols. This procedure usually provides good reinforcing filler, however, depending upon the acidity level and the presence of contaminants such as iron, polymeric gum balls can be formed in the filler. The conditions of treatment can also lead to particulate contamination due to vessel corrosion. As a consequence of the above, reproducibility of the treatment is poor. Further, the treatment of filler by this manner and incorporation of the same into polysiloxane rubbers is a cumbersome process which requires both treatment and blending facilities.
Accordingly, it is an object of the present invention to simplify the treatment of silica filler and the incorporation of the same in to polysiloxane rubbers.
It is a further object of the present invention to produce silica filled polysiloxane rubber having the desired levels of physical properties at reduced levels of silica filler content.
It is a further object of the present invention to combine the surface treatment process for silica filler and the incorporation of silica filler into polysiloxane rubber into a single step.
It is still a further object of the present invention to produce a heat age stable halogenated alkyl or aryl substituted polysiloxane rubber wherein silica filler is surface treated in situ.